Long-term release of carbon from grassland soil amended with different slurry particle size fractions: a laboratory incubation study

Application of animal manure to agricultural soils enhances both native soil carbon (C) and overall (native soil C and added C) respiration. CO2 effluxes were measured in a laboratory incubation study for 1465 days after the application of different slurry fractions (>2000, 425-2000, 250-425, 150-250, 45-150 and <45 mu m) to a grassland soil. The slurry-derived C present in the soil was traced using the natural abundance delta C-13 method. We used two kinetic (single and two pool) models to fit the experimental data and to test the model validity with respect to long-term data sets. Mean residence times (MRTs) of the particle size based slurry-C fractions were estimated using these models and a linear C-13 natural abundance based approach. The results showed that slurry-C degradation in soil over time varied between the different particle size based slurry treatments. The two kinetic soil-C models were successful to predict medium-to long-term carbon release from soil amended with animal slurry. The estimated MRTs did vary between the linear (3.8-5.6 years) and non-linear based (0.8-3.8 years) (model) approaches. Slurry-derived C could still be (isotopically) detected in the soil 4 years after slurry application using the natural abundance delta C-13 method. This suggests that it may take a decadal timescale or longer before the entire amount of C introduced through whole slurry amendments to grassland soils is fully dissipated. Copyright (C) 2011 John Wiley & Sons, Ltd.